LeetCode 103. Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3   / \  9  20    /  \   15   7

return its zigzag level order traversal as:

[  [3],  [20,9],  [15,7]]

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {        List<List<Integer>> list = new ArrayList<>();        if (root == null) return list;        Queue<TreeNode> queue = new LinkedList<>();        queue.offer(root);                int index = 1;        while (!queue.isEmpty()) {            int n = queue.size();            LinkedList<Integer> tmpList = new LinkedList<>();            for (int i = 0; i< n; i++) {                if (queue.peek().left != null) queue.offer(queue.peek().left);                if (queue.peek().right != null) queue.offer(queue.peek().right);                //先左后右                if (index % 2 == 1) {                                     tmpList.add(queue.poll().val);                } else {                    tmpList.addFirst(queue.poll().val);                }            }            if (index == 1) index = 2;            else index = 1;            list.add(tmpList);        }        return list;    }}